Centrifugal compressors are known to utilize diffusers for converting a portion of the kinetic energy of a working fluid leaving a compressor wheel into static pressure by slowing the flow velocity of the working fluid through an expanding flow volume region. Diffusers may incorporate airfoils, commonly called vanes, for directing the working fluid through the expanding volume to enhance this process, with each vane having a pressure side and a suction side relative to an angle of attack of the incoming working fluid. FIG. 1 illustrates how a prior art diffuser 10 may develop a large flow separation zone 12 on the suction side 14 of a diffuser vane 16 under certain conditions. The flow separation zone 12 is essentially a flow boundary layer that has a lower velocity than the remainder of the flow and therefore hinders the overall fluid flow rate. The flow separation zone 12 creates a distorted exit flow 18 from the compressor, reducing the efficiency of the compressor and potentially leading to surge and stall of the compressor, with resultant damage to the compressor and/or a downstream turbocharged engine. For the embodiment of a compressor used as a turbocharger for the diesel engine of a railroad locomotive, the compressor is most vulnerable to such surge and stall events when the locomotive is operating at high altitude, low ambient temperature, and high manifold air temperature; for example when just exiting a high altitude tunnel.
As illustrated in FIG. 1, the conventional wisdom for the design of compressor diffuser vanes 16 is to provide uninterrupted surfaces 20 from the leading edge 22 to the trailing edge 24 of the vanes to maximize the surface area of the vane exposed to the differential pressure between the suction side 14 and the pressure side 26. The position and angle of the vane is chosen as a compromise between avoiding stalling of the flow and maintaining efficient pressure recovery for the angles of attack of the various incoming air flow streams that were anticipated to impinge upon the vane.